This will be disputed, but the original 222 1/2 was just a 222 reamer, run into 222 mag length with no other changes. To form, just space a 222 rem FL die .150" off the shell holder & push back the shoulder on a 222 mag case.
However, there are many variations in shoulder angle, etc. I still have a version that is a 222 mag reamer run into 223 length to give (kind of) a 222 1/2 with a shorter neck. All the versions shoot well, but we're talking early 1970's technology. With the move to for-score matches, even the 6's are at a disadvantage.
Regards, Ron